This invention relates to a mass spectrograph of the type for ionizing a sample under a relatively near atmospheric condition of pressure such as an inductively coupled plasma mass spectrograph (ICP-MS), an electro spray mass spectrograph (ES-IMS) or an atmospheric pressure chemical ionization mass spectrograph (APCI-MS).
A prior art ESI-MS is shown schematically in FIG. 4 and an portion thereof around its skimmer is shown enlarged in FIG. 5. This mass spectrograph is provided with a first intermediate chamber 12 and a second intermediate chamber 15 between an ionization chamber 10 having a nozzle 11 connected to the outlet of the column of a liquid chromatographic apparatus and an analyzing chamber 18 with a quadrupole filter 19 and an ion detector 20, each being mutually separated by a partition wall. The ionization chamber 10 and the first intermediate chamber 12 are connected only through a heated capillary of a small inner diameter serving as a solvent-removing pipe 13. The first intermediate chamber 12 and the second intermediate chamber 15 are connected only through a conically shaped skimmer 16 having an orifice 16a of a small diameter at its tip.
The interior of the ionization chamber 10 is nearly in the atmospheric condition due to the gasified molecules of the sample liquid continuously supplied thereinto through the nozzle 11. The interior of the first intermediate chamber 12 is at a low vacuum condition of about 102 Pa by means of a rotary pump (RP). The interior of the second intermediate chamber 15 is at a medium vacuum condition of about 10xe2x88x921-10xe2x88x922 Pa by means of a turbo-molecular pump (TMP). The interior of the analyzing chamber 18 is at a high vacuum condition of about 10xe2x88x923-10xe2x88x924 Pa by means of another turbo-molecular pump (TMP). In other words, the degree of vacuum increases as one moves from one chamber to the next, starting at the ionization chamber 10 towards the analyzing chamber 18 such that the interior of the analyzing chamber 18 is maintained at a high vacuum condition.
A sample liquid is sprayed (or electro-sprayed) through the nozzle 11 into the ionization chamber 10, and the sample molecules are ionized while the solvent contained in the liquid drops is evaporated. Small liquid droplets with ions mixed in are pulled into the solvent-removing pipe 13 due to the pressure difference between the ionization chamber 10 and the first intermediate chamber 12. As they pass through the solvent-removing pipe 13, the solvent is evaporated and the process of ionization proceeds further. A pair of mutually facing planar electrodes or a ring-shaped electrode 14 is provided inside the first intermediate chamber 12. The electric field generated by this electrode 14 serves not only to pull in the ions through the solvent-removing pipe 13 but also to converge the ions to a point (xe2x80x9cbackward focal pointxe2x80x9d) F near the orifice 16a of the skimmer 16.
The converged ions are caused to pass through the orifice 16a of the skimmer 16 by the pressure difference between the first intermediate chamber 12 and the second intermediate chamber 15 and is directed into the analyzing chamber 18 after being converged and accelerated by means of an ion guide 17 (also referred to as the ion lens or the ion-transporting lens). Inside the analyzing chamber 18, only those of the ions having a specified mass number (the ratio of mass m to charge z) are passed through the longitudinal space at the center of the quadrupole filter 19 and reach the ion detector 20 to be detected thereby.
The function of the ion guide 17 is to accelerate flying ions while causing them to be converged. Ion guides with many different shapes have been proposed. The so-called multi-pole type is one of known types, having a plurality of approximately cylindrically shaped rod electrodes arranged so as to circumscribe a circle of diameter d1 and mutually separated and having a voltage difference superposing high-frequency voltages with phases mutually inverted by a same direct-current voltage applied between each mutually adjacent pair of these rod electrodes. Such a high-frequency electric field causes the ions introduced in the direction of the optical axis C to move forward while vibrating at a specified frequency. As a result, the ions can be converged more effectively and more ions can be sent into the analyzing chamber 18 on the downstream side.
For the purpose of passing ions as efficiently as possible through the first intermediate chamber 12 and the second intermediate chamber 15, it is desirable to reduce the distance as much as possible between the orifice 16a and the space surrounded by the rod electrodes of the ion guide 17. For this reason, the end surface of the ion guide 17 facing the skimmer 16 is formed with a slope so as to match the sloped surface of the skimmer 16 and the ion guide 17 is disposed such that its sloped end surface protrudes into the conically shaped portion of the skimmer 16. This makes it time-consuming to fabricate the rod electrodes, affecting the production cost adversely.
Another problem is that the orifice 16a of the skimmer 16 and its neighboring parts become contaminated with sample ions that stick to them, and the skimmer 16 must therefore be designed to be detachable. With the skimmer 16 and the ion guide 17 as formed above, either of them should be made slidable in the direction of the aforementioned optical axis C or the skimmer 16 must be attached to be rotatable by means of a hinge. This causes the attachment mechanism of the skimmer 16 and the ion guide 17 to be complicated.
It is therefore an object of this invention in view of the problems described above to provide a mass spectrograph having an ion guide with a simplified structure and a simplified attachment mechanism for the skimmer while maintaining a high level of efficiency in passing ions.
A mass spectrograph embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as being of the kind having an ionization chamber for ionizing a sample, a skimmer in a conical shape with an orifice, an analyzing chamber at a lower pressure than inside the ionization chamber such that the generated ions are pulled through the orifice into the analyzing chamber, and a multi-pole ion guide which is disposed immediately behind the skimmer and comprised of an even number of cylindrically shaped electrodes all elongated in an axial direction but also wherein these electrodes are disposed so as to circumscribe an inscribed circle and the bottom surface of the conically shaped skimmer has a smaller diameter than the inscribed circle of the ion guide such that the ions can reach the analyzing chamber more efficiently.